With the permeation of information technology (IT) environment including the Internet, various information is circulated throughout the world.
Furthermore, information circulation using IT is advancing in every scene from business to personal hobbies and avocational activities. Circulating information has a high degree of flexibility, and is diverse in format and content.
There are some kinds of information having various risks; for example, there are information having a risk of becoming a problem in case of leakage of its existence, information having a harmful impact such as an attack or discomfort on a recipient of information circulated, etc. Such information is always circulated in a hidden way through IT. Furthermore, there is a possibility that people unknowingly perform an action which will become a problem; in some cases, such an action may develop into a major problem or a crime before people realize it. Moreover, once information comes into the world, the information is not able to be withdrawn. In this way, circulation of information may damage individual or company's reliability.
Electronic mail (hereinafter, sometimes referred to as “mail”) is cited as a technology used for information circulation in a company or the like. In recent years, there is used a technique to register keywords that impact a mail recipient and frequently appearing keywords in advance and extract electronic mails including any of the keywords.
Patent Literature 1: Japanese Laid-open Patent Publication No. 2005-284454
Patent Literature 2: Japanese Laid-open Patent Publication No. 2007-249584
Patent Literature 3: Japanese Laid-open Patent Publication No. 2000-132553